This invention relates generally to apparatus for recording and reproducing information signals and, more particularly, is related to apparatus for recording and reproducing digital video signals in a plurality of record tracks extending obliquely on a magnetic tape.
Generally, a video tape recorder (VTR) includes at least one rotary magnetic head which records and/or reproduces video signals from a magnetic tape, with the magnetic head or heads being mounted on a rotary portion of a tape guide drum about which the tape is at least partially wrapped. However, various restrictions must be taken into consideration when constructing a VTR, for example, the number of magnetic heads, the tape wrapping angle about the guide drum, and the like.
For example, where one rotary magnetic head is provided for recording and reproducing a video signal with respect to the magnetic tape, the timing of the video signal which is recorded and reproduced can not be freely varied because of the restrictions of a one head arrangement. Thus, with a VTR having a single rotary magnetic head, if synchronizing signals are not recorded on the magnetic tape, synchronizing signals must be added to the video signal during reproduction in the interval during which the rotary magnetic head does not contact the magnetic tape so as to compensate for the synchronizing signals that were not recorded. As a result, a constant restriction in the timing of the signal recorded by the rotary magnetic head exists in the interval during which the rotary magnetic head contacts the magnetic tape so that the wrapping angle of the magnetic tape on the tape guide drum can not be freely varied.
On the other hand, with a VTR having two rotary magnetic heads positioned 180.degree. apart, that is, diametrically opposite to each other, the wrapping angle of the magnetic tape around the tape guide drum is 180.degree., thereby reducing the wrapping angle in comparison with that for a one head arrangement and providing the advantage that loading of the magnetic tape around the guide drum is easier. In such case, a tape cassette containing the magnetic tape can be readily used with the VTR. It is to be appreciated that, with the two head arrangement, the two rotary magnetic heads are selectively switched ON and OFF, or changed over, every 180.degree. so that the information signals recorded on and reproduced from the magnetic tape are continuous in time. Except for the fact that a margin is provided at both edges of the magnetic tape so as to avoid adverse affects when each rotary magnetic head first contacts and terminates its contact with the magnetic tape, there are no direct advantages in increasing the wrapping angle of the magnetic tape around the tape guide drum. Accordingly, the wrapping angle of the magnetic tape is selected to be approximately 180.degree..
On the other hand, in a VTR equipped with two or more pairs of rotary magnetic heads, the timing of the information signal recorded on the magnetic tape can be freely varied so that it is unnecessary to specify a specific wrapping angle, for example, 360.degree. (340.degree.), 180.degree. and 90.degree. for two or more pairs of rotary magnetic heads. In such case, however, an optimum wrapping angle should be chosen which optimally satisfies various conditions. Thus, the wrapping angle should be chosen taking into consideration the following conditions:
(1) The number of rotary magnetic heads and the number of channels with respect to the rotary transformer must be as few as possible. PA1 (2) The friction generated when the magnetic tape is wrapped around the tape guide drum must be sufficiently small and non-uniformity in contact of each rotary magnetic head with the magnetic tape, caused by, for example, sticking of the tape to the tape guide drum, non-uniformity or scattering in the air layer, and the like, must be minimized. PA1 (3) The magnetic tape must be easily loaded around the tape guide drum with the magnetic tape being conveniently contained in a tape cassette. PA1 (4) A large amount of information in a narrow frequency band must be capable of being processed between the rotary magnetic heads and the rotary transformer.
In regard to condition (1), the fewest number of rotary magnetic heads is, of course, one. However, as previously discussed in regard to the one head arrangement, the timing of the information signals recorded on the magnetic tape is severely restricted, and the variety of signals that can be processed are also considerably restricted. In other words, a single rotary magnetic head system is not suitable for recording video information in digital form. Further, in accordance with present sampling standards for digital video signals, when the sampling frequencies are 13.5 MHz for the luminance signal and 6.75 MHz for the two color difference signals, 8 bits per sample are selected. In such case, the bit rate for the digital video signal, including the blanking period, is 216 Mbits/sec, thereby requiring at least a 4-head and 2-channel system. In such system, two pairs of rotary magnetic heads are positioned apart with an angular spacing of 180.degree. and with each pair of rotary magnetic heads being formed of two adjacent rotary magnetic heads having magnetic gaps with different azimuth angles. It is to be appreciated that, in such case, it is possible that the recording and playback rotary magnetic heads are separately provided, or alternatively, that the rotary magnetic heads serve both for recording and playback.
In this regard, the 4-head and 2-channel system can be satisfactorily used for the high 216 Mbits/sec rate, while also providing a tape wrapping angle of 180.degree., thereby enabling easy loading of the magnetic tape around the tape guide drum. Thus, except for the 4-head and 2-channel system, other systems do not meet the aforementioned conditions (1) and (4). The following Table compares the 4-head and 2-channel system with other systems as to the tape wrapping angle and Nyquist frequency.
TABLE ______________________________________ Number of Tape Wrapping Nyquist Number of Heads Channels Angle Frequency ______________________________________ 1 1 360.degree. 108 MHz 2 1 180.degree. 108 MHz 2 2 360.degree. 54 MHz 3 1 120.degree. 108 MHz 3 3 360.degree. 36 MHz 4 1 90.degree. 108 MHz 4 2 180.degree. 54 MHz 4 4 360.degree. 27 MHz ______________________________________
However, as shown in the above Table, the 4-head and 2-channel system requires a significantly high Nyquist or minimun sampling frequency of 54 MHz. Considering the frequency characteristics of the rotary magnetic heads, the rotary transformer and the recording and reproducing amplifiers, it is thereby quite difficult to put the 4-head and 2-channel system into practice. In view of these problems, it is desirable to provide a VTR operable with a lower Nyquist frequency, yet capable of providing a high signal-to-noise (S/N) ratio with a sufficiently wide frequency range, while also not increasing the diameter of the tape guide drum.